1. Field of the Invention
The present invention relates to a surface acoustic wave filter, this filter being a filter whose acoustic path has been folded back.
2. Description of the Related Art
A filter of this type generally includes, on a single substrate, at least one input transducer, at least one output transducer, both bi-directional and both placed on this substrate so as to define, in respect of the surface acoustic waves, a longitudinal direct path between these two transducers, at least two reflecting gratings or "reflectors" being moreover provided respectively on this substrate on either side of this pair of transducers and on this longitudinal axis of propagation of the surface acoustic waves, so that the centres of transduction of the transducers lie on antinodes of the resonant cavity.
In this regard, a "two pole" filter with longitudinally coupled resonant cavities of the prior art is represented diagrammatically in FIG. 1 attached.
This filter includes, on a single substrate 1 and aligned with the longitudinal axis Y of propagation of the surface acoustic waves symmetrically with respect to the inter-transducer mid-line (L):
an input transducer 2 which receives the microwave signal applied between the electrical input terminals 3 of the filter and which transforms this signal into surface acoustic waves of like frequency moving, in one and the opposite direction, over the substrate 1 and in the longitudinal direction Y; PA1 An output transducer 4, identical to the transducer 2, and capable conversely of transforming the surface acoustic waves into electrical signals extracted on the electrical output terminals 5 of the filter. PA1 a first resonant cavity defined by the two reflectors 6 and 8, PA1 a second resonant cavity defined by the two reflectors 7 and 8.
Two identical "outer" reflecting gratings 6 and 7 placed respectively, and symmetrically, on either side of the substrate 1. These gratings are conventionally formed by a periodic or quasi-periodic grating of elementary reflectors, the period between reflecting elements being an integer multiple of half-wavelengths at the central frequency of operation of the filter.
A "central" reflecting grating 8 provided between the two transducers 2, 4, along the Y axis and in the middle of the interval separating these two transducers, the distances X between each of these transducers and the closest edge of this central reflector 8 being equal. This grating 8 has the same periodicity as the outer gratings 6, 7. On the other hand, it exhibits a non-negligible transmission coefficient, unlike these outer gratings which are in practice reflecting only. These reflecting gratings (outer and central) may as appropriate be weighted to improve their off-band response (increase in rejection).
Outside the "stop band" of the filter (that is to say the frequency band in which the gratings are reflecting), the device behaves like a simple delay line, only the direct path between the input transducer 2 and the output transducer 4 being taken into account.
In the stop band, this direct path still exists, but furthermore the device operates as two longitudinally coupled resonant cavities:
The electromechanical coupling with these resonant cavities is obtained through the input/output transducers 2, 4. Depending on the value of this coupling, the magnification from the cavities is greater or smaller and consequently the relative bandwidth of the device lower or higher. Similarly, the coupling between the two cavities (that is to say the transmission from the central grating) must be matched to the relative bandwidth to be produced.
Thus, when this coupling is weak, the number of outward/return paths between the gratings is large, the energy remaining confined in the cavities for a long time. The impulse response is lengthy and the relative bandwidth low: operation is that of a resonator with high magnification factor.
Conversely, when this coupling is strong, a sizeable fraction of energy is absorbed by the transducers with each passage of the surface acoustic wave. The impulse response is then short and the relative bandwidth large: operation is that of a resonator with low magnification factor.
Note that in the absence of central reflector 8, operation is that of a filter with a single resonant cavity defined by the two outer reflectors 6, 7: we then have a "one pole" filter, whilst the device of FIG. 1 is a "two pole" filter, with steeper flanks.
In the case of this kind of surface wave filter with at least one resonant cavity, the presence of the direct path causes the appearance of side lobes whose level is all the higher when it is desired to widen the relative pass band of the filter. The presence of these side lobes renders this kind of filter practically unusable for wide relative bandwidths.